1. Field of the Invention
This invention relates to a method for forming a functional film, particularly a deposited film which is useful for semiconductor devices or photosensitive devices for electrophotography.
2. Description of the Prior Art
For formation of a silicon-containing deposited film such as a hydrogenated, halogenated or halo-containing hydrogenated amorphous silicon film (these are referred to comprehensively hereinafter as "a-Si(H,X)") or a polycrystalline silicon film, there have been attempted the vacuum deposition method, the plasma CVD method, the CVD method, the reactive sputtering method, the ion plating method and the optical CVD method. Among them, the plasma CVD have been generally employed and practiced in industry.
However, silicon-containing deposited films are further required to be improved in overall characteristics, with respect to electrical and optical characteristics, fatigue characteristics in repeated uses or environmental characteristics, and further productivity and bulk production, including uniformity and reproducibility.
The reaction process in formation of a-Si(H,X) deposited films according to the generally practiced plasma CVD method is considerably more complicated as compared with the CVD method of the prior art, and its mechanism has not been completely elucidated. There are a number of parameters for formation of the deposited film (e.g. substrate temperature flow rates and ratios of gases introduced, pressure during formation, high frequency power, electrode structure, structure of reaction vessel, evacuation speed, plasma generation system, etc.) and formation depends on a large number of these parameters. Accordingly, the plasma may sometimes become unstable, whereby marked deleterious effects have not scarecely been incurred on the deposited film. Moreover, the parameters inherent in the device must be chosen individually for respective devices and therefore the preparation conditions can be generalized with difficulty under the present situation.
On the other hand, in order to have a silicon-containing deposited film capable of exhibiting electrical and optical characteristics which can satisfy sufficiently the respective uses, it is now accepted as the best to form the deposited film according to the plasma CVD method.
However, depending on the uses for which the silicon-containing deposited film is applied, it is required to effect bulk production with good reproducibility, while satisfying fully enlargement of area, uniformity of film thickness and uniformity of film quality. For this reason, in formation of the silicon-containing deposited film according to the plasma CVD method, an enormous amount of installation investment becomes necessary for bulk production device, and items for controlling such production will become also increased in number and tolerance in control will also become narrower, requring also a severe control of the device. These points have been pointed out as the problems to be improved. On the other hand, in the prior art technique according to the thermal CVD method, a high temperature is required and no deposited film having practically useful characteristics has been obtained.
As described above, in formation of a silicon-containing deposited film, it has been earnestly desired to develop a method for forming films suitable for bulk production at low cost with keeping practically useful characteristics and uniformity. The same is true with other functional films such as silicon nitride film, silicon carbide film, silicon oxide film, etc.